Micro-scale sensors, and among them those based on piezoelectric quartz crystals, are devices used to accurately measure variations in the weight deposited thereon per unit area, through the changes withstood by the resonance frequency of such crystals operating as resonators. Among the variety of microbalance sensors currently existing on the market, the so-called AT cut quartz resonators (wherein this type of cut corresponds to a cut at an angle with an inclination of 35°15′ with respect to the optical axis z of the crystal and perpendicular to the plane y-z thereof) are becoming an alternative analytical tool in a wide range of implementations, in which one wishes to detect the presence of species in solution or to characterize chemical processes, with a resolution comparable, in many cases, to the classical chemical techniques (See references: A. W. Czanderna and C. Lu (1984) in “Applications of piezoelectric quartz crystal microbalances”, C. LU and A. W. Czanderna (eds), Elsevier, Amsterdam, Vol. 7; A. Janshoff, H-J Galla and C. Steinem (2000) “Piezoelectric mass-sensing devices as biosensors—an alternative to optical biosensors?” Angew. Chem Int. Ed. 39:4004-4032; MA. Cooper and V T. Singleton (2007) “A survey of the 2001 to 2005 quartz crystal microbalance biosensor literature: applications of acoustic physics to the analysis of biomolecular interactions” Journal of Molecular Recognition 20 (3): 154-184; T A. Carnesano, Y T. Liu and M. Datta (2007) “Measuring bacterial adhesion at environmental interfaces with single-cell and single-molecule techniques” Advances in Water Resources 30 (6-7):1470-1491; 0. Lazcka, F J. Del Campo and F X, Muñoz (2007) “Pathogen detection: A perspective of traditional methods and biosensors” Biosensors & Bioelectronics 22 (7):1205-1217; TS. Hug (2003) “Biophysical methods for monitoring cell-substrate interactions in drug discovery” Assay and Drug Development Technologies 1 (3): 479-488; FL. Dickert, P. Lieberzeit and O. Hayden (2003) “Sensor strategies for micro-organism detection—from physical principles to imprinting procedures” Analytical and Bioanalytical Chemistry 377 (3):540-549; K A. Marx (2003) “Quartz crystal microbalance: A useful tool for studying thin polymer films and complex biomolecular systems at the solution-surface interface” Biomacromolecules 4 (5): 1099-1120; K A. Fahnrich, M. Pravda and G G. Guilbault (2002) “Immunochemical detection of polycyclic aromatic hydrocarbons (PAHs)” Analytical Letters 35 (8): 1269-1300; J. Wegener, A Janshoff and C. Steinem (2001) “The quartz crystal microbalance as a novel means to study cell-substrate interactions in situ” Cell Bio-chemistry and Biophysics 34 (1):121-151; C K. O'Sullivan and G G. Guilbault “Commercial quartz crystal microbalances—theory and applications” Biosensors & Bioelectronics 14 (8-9):663-670; C K. O'Sullivan, R. Vaughan and G G. Guilbault (1999) “Piezoelectric immunosensors—theory and applications” Analytical Letters 32 (12):2353-2377; K. Bizet, C. Grabielli and H. Perrot (1999) “Biosensors based on piezoelectric transducers” Analysis EurJAC 27:609-616).
The use of the AT cut quartz crystal resonator as quartz crystal microbalance, better known by its initials in Anglo-Saxon literature as QCM (quartz crystal microbalance), is based on the well known, Sauerbrey equation (G. Sauerbrey (1959) “Verwendung von schwingquarzen zur wägung dünner Schichten and zur mikrowägung” Zeitschrift Fuer Physik 155 (2): 206-222), by those skilled in the art. Sauerbrey's equation states that the decrease in the resonance frequency of the resonator is proportional to the increase in the surface density in weight of the coating on the surface of the sensor. When the sensor is in contact with a Newtonian liquid medium, Kanazawa's equation (K.K. Kanazawa and J. G. Gordon II (1985) “The oscillation frequency of a quartz resonator in contact with a liquid” Analytica Chimica Acta 175:99-105) provides the shift in the resonance frequency of the resonator due to contact with the fluid. For a QCM sensor with one of the surfaces coated with a very fine layer of material, so thin that the lag in the acoustic wave through the coating thickness is very small, and exposed to Newtonian liquid. Martin's equation (I) provides the quantitative connection of the combination of the effects of the coating weight (Sauerbrey effect) and the liquid (Kanazawa effect) in the variation of the resonance frequency (S. J. Martin, V. E. Granstaff and G. C. Frye (1991) “Characterization of quartz crystal microbalance with simultaneous mass and liquid loading” Anal. Chem. 63:2272-2281).
                              Δ          ⁢                                          ⁢          f                =                              -                                          2                ⁢                                  f                  s                  2                                                            Z                cq                                              ⁢                      (                                                            ρ                  c                                ⁢                                  h                  c                                            +                                                1                  2                                ⁢                                  ρ                  L                                ⁢                                  δ                  L                                                      )                                              (        I        )            
In the above equation, the first term of the second member corresponds to the Sauerbrey effect and the second one to the Kanazawa effect, wherein fs is the resonance frequency of the sensor, Zcq is the characteristic acoustic impedance of the quartz, ρc and hc are, respectively, the density and thickness of the coating; and ρL and δL are, respectively, the density and the depth of penetration of the acoustic wave in the liquid: ½ρLδL is, in fact, the surface density of equivalent weight associated with the oscillating movement of the surface of the sensor in contact with the liquid medium.
According to equation (I), for a certain surface weight density of the coating, the absolute value of the frequency offset increases in direct proportion to the square of the resonance frequency. Consequently, it seems logical to think that a much greater sensitivity will have a QCM sensor the higher its resonance frequency is. In fact, the resonance frequency has always been the fundamental characterization parameter in QCM sensors.
Indeed, in practice, the vast majority of techniques used in the characterization of QCM sensors have been used to determine the variation in the resonance frequency of the resonator, and other relevant parameters thereof (the U.S. Pat. No. 5,201,215 granted to Granstaff et al. “Method for simultaneous measurement of mass loading and fluid property changes using a quartz crystal microbalance”, includes other parameters of the sensor that should be monitored; see also references: A. Arnau, V. Ferrari, D. Soares, H. Perrot, “Interface Electronic Systems for AT-cut QCM Sensors. A comprehensive review”, in Piezoelectric Transducers and Applications, 2nd Ed., pp. 117, A. Arnau Ed., Springer-Verlag Berlin Heidelberg, (2008); F. Eichelbaum, R. Borngräber, J. Schröder, R. Lucklum, and P. Hauptmann (1999) “Interface circuits for quartz crystal microbalance sensors,” Rev. Sci Instrum. 70:2537-2545): network or impedance analyzers are used to determine the conductance of the resonator in the range of resonance frequencies and determine the frequency that corresponds with the maximum conductance (J. Schröder, R. Borngräber, R. Lucklum and P. Hauptmann (2001) “Network analysis based interface electronics for quartz crystal microbalance” Review Scientific Instruments 72 (6):2750-2755; S. Doerner, T. Schneider, J. Schröder and P. Hauptmann (2003) “Universal impedance spectrum analyzer for sensor applications” in Proceedings of IEEE Sensors 1, pp. 596-594); the art of decay, which is contained in U.S. Pat. No. 6,006,589 granted to Rodahl et al., in 1999 (see also reference M. Rodahl and B. Kasemo (1996) “A simple setup to simultaneously measure the resonant frequency and the absolute dissipation factor of a quartz crystal microbalance” Rev. Sci Instrum. 67:3238-3241), processes the resulting signal by disconnecting the signal with which the resonator has been excited for a certain time, at a frequency close to that of the resonance. This analysis ultimately provides information on the variation of the resonance frequency, series or parallel depending on the configuration, and the losses in the resonator; in the techniques based on oscillators the resonant sensor is used as an element for controlling the frequency of oscillation, allowing a continuous monitoring of a frequency that corresponds to a specific phase of the resonator in the resonance range. This frequency can be used in many applications such as reference to the resonance frequency of the resonator (see the following references: H. Ehahoun, C. Gabrielli, M. Keddam, H. Perrot and P. Rousseau (2002) “Performances and limits of a parallel oscillator for electrochemical quartz crystal microbalances” Anal Chem. 74:1119-1127; C. Barnes (1992) “Some new concepts on factors influencing the operational frequency of liquid-immersed quartz microbalances” Sensors and Actuators A-Physical 30 (3): 197-202; K. O. Wessendorf (1993) “The lever oscillator for use in high resistance resonator applications” in Proceedings of the 1993 IEEE International Frequency Control Symposium, pp. 711-717; R. Borngräber, J. Schröder, R. Lucklum and P. Hauptmann (2002) “Is an oscillator-based measurement adequate in a liquid environment?” IEEE Trans. Ultrason. Ferroelect. Freq. Contr. 49 (9): 1254-1259; S. J. Martin, J. J. Spates, K. O. Wessendorf, T. W. Schneider and R. J. Huber (1997) “Resonator/oscillator response to liquid loading” Anal. Chem. 69:2050-2054). The techniques based on oscillators are the simplest and quickest for monitoring the frequency, but have operating disadvantages in liquid medium, wherein numerous applications of great interest taken place; for such reason large efforts have been made for designing oscillators suitable for these applications, which have resulted in different patents such as: U.S. Pat. No. 4,783,987 granted to Hager in 1988 titled “System for sustaining and monitoring the oscillation of piezoelectric elements exposed to energy-absortive media”; U.S. Pat. Nos. 4,788,466 and 6,848,299 B2 granted to Paul et al., in 1988 and 1995, “Piezoelectric sensor Q loss compensation” and “Quartz crystal microbalance with feedback loop for automatic gain control”; U.S. Pat. Nos. 5,416,448 and 6,169,459 granted to Wessendorf in 1995 and 2001 “Oscillator circuit for use with high loss Quartz resonator sensor” and “Active bridge oscillator”; finally there is a group of techniques that could be so-called “hooking techniques” (see the references A. Arnau, T. Sogorb, Y. Jiménez (2002) “Circuit for continuous motional series resonant frequency and motional resistance monitoring of quartz crystal resonators by parallel capacitance compensation” Rev. Sci. Instrum. 73 (7): 2724-2737; V. Ferrari, D. Marioli, and A. Taroni (2001) “Improving the accuracy and operating range of quartz microbalance sensors by purposely designed oscillator circuit” IEEE Trans. Instrum. Meas. 50:1 1 19-1 122; A. Arnau, J. V. García, Y. Jiménez, V. Ferrari and M. Ferrari (2007) “Improved Electronic Interfaces for Heavy Loaded at Cut Quartz Crystal Micro-scale Sensors” in Proceedings of Frequency Control Symposium Joint with the 21 st European Frequency and Time Forum. IEEE International, pp. 357-362; M. Ferrari, V. Ferrari, D. Marioli, A. Taroni, M. Suman and E. Dalcanale (2006) “In-liquid sensing of chemical compounds by QCM sensors coupled with high-accuracy ACC oscillator” IEEE Trans. Instrum. Meas. 55 (3):828-834; B. Jakoby, G. Art and J. Bastemeijer (2005) “A novel analog readout electronics for microacoustic thickness shear-mode sensors” IEEE Sensors Journal 5 (5):1106-1111; C. Riesch and B. Jakoby (2007) “Novel Readout Electronics for Thickness Shear-Mode Liquid Sensors Compensating for Spurious Conductivity and Capacitances” IEEE Sensors Journal 7 (3): 464-469) which can be considered as sophisticated oscillators, because these include a feedback loop, wherein the sensor exciting source can be considered external thereto and wherein the feedback condition of the loop can be accurately calibrated. These techniques allow accurately monitoring the dynamic series resonance frequency of the resonator and some of them have been protected by patents (MI2003A000514, granted to Ferrari et al, “Metodo e dispositivo per determinare la frequenza di risonanza di sensori piezoelettrici risonati” and patent ES2197796 granted to Arnau et al., in 2004 “Sistema de caracterización de sensores de cristal de cuarzo resonante en medios fluidos, y procedimiento de calibración y compensación de la capacidad del cristal de cuarzo”.
Other recent patents using, in one way or another, some of the described techniques or variations thereof but with a common objet that is monitoring the resonance frequency of the sensor have been reviewed (those granted to J. P. Dilger et al., in 2000 and 2001, U.S. Pat. No. 6,161,420, “High frequency measuring circuit” and U.S. Pat. No. 6,222,366 B1 U.S. “High frequency measuring circuit with Inherent noise reduction for chemicals resonating sensors”; that granted to J. R. Vig in 2001, U.S. Pat. No. 6,247,354 B1, “Techniques for sensing the properties of fluids with resonators”; the patent granted to Chang et al., in 2003, U.S. Pat. No. 6,557,416 B2 “High resolution biosensor system”; the patent granted to Nozaki in 2006, U.S. Pat. No. 7,036,375 B2, “QCM sensor and QCM sensor device”; that granted to Dayagi et al., in 2007, U.S. Pat. No. 7,159,463 B2 “Sensitive and selective method and device for the detection of trace amounts of a substance”; that granted to Itoh et al., in 2007, U.S. Pat. No. 7,201,041 B2 “Analysis method using piezoelectric resonator”; that granted to Zeng et al., in 2008, U.S. Pat. No. 7,329,536 B2 “Piezoimmunosensor”).
The main reason to perform the monitoring of the resonance frequency of the resonator and, therefore, its variation is the existence of a simple connection between this variation and the physical quantities of interest in a real application. In this case the variation in weight surface density on the surface of the sensor, which may be due to changes in the density of the coating or the properties of the liquid media, has been presented in equation (I). In many applications, for example, in piezoelectric biosensors, covering a wide range of process characterization (see reference M A. Cooper and V T. Singleton (2007) “A survey of the 2001 to 2005 quartz crystal microbalance biosensor literature: applications of acoustic physics to the analysis of biomolecular interactions” Journal of Molecular Recognition 20 (3):154-184), the displacements experienced by the resonance frequency of the sensor are usually very small, in the neighborhood of tens of hertz in megahertz, and are due to the increase in weight of the sensitive thin layer covering the resonator, wherein the liquid medium substantially maintains its fluid physical properties constant. Therefore, great efforts are being made to improve the sensitivity of the quartz crystal microbalance sensor; most of these efforts are aimed at increasing the resonance frequency of the resonator, as suggested by equation (I). However, equation (I) provides a theoretical ideal sensitivity that implicitly assumes an infinite stability of the components of the system for characterization and the measurement process, such that there are no disturbances associated with the measurement system or instabilities coming from the electronic system for characterization. Unfortunately this is not so, and the sensitivity does not exclusively depend on the resonator, but also on the design and configuration of the measurement system and the characterization electronic circuit. The entire infrastructure required for performing the experiment, including the measurement cell, flow elements, pumps, systems for adjusting the temperature, etc., except the characterization electronic circuit are understood here as the measurement system. Assuming that the measurement system has been designed to minimize disruptions and interferences that can affect the resonance frequency of the resonator such as: temperature changes, vibration, changes in the pressure of the fluid by the use of inadequate injection pumps, etc., the sensitivity of the assembly will depend on the accuracy of the measurement of the resonance frequency of the sensor which, in turn, will depend on the interference generated by the electronic system for characterization itself. Therefore, the sensitivity cannot be adequately assessed without considering the system used to characterize the sensor.
The systems used to characterize the piezoelectric resonators in microbalance applications, most of which have been described above, can be classified into two types: a) those that passively interrogate the sensor kept outside the characterization system, and b) those in which the sensor is part of the characterization system itself. The network or impedance analyzers and the techniques of decay are in the first group, whereas the second group may include the oscillators. The hooking techniques can be considered to be found between both groups.
The advantages of network or impedance analyzers are recognized and are associated with the fact that the sensor can be characterized after a calibration wherein any electrical influence external to the sensor itself has been balanced. Decay methods provide high accuracy, as long as the precision in the acquisition of the decay signal is high, both in phase and amplitude, resulting complex for high-frequency resonators. Thus, for high frequency resonators, higher than 50 MHz, only the impedance analyzers are accurate enough, but the high cost and size thereof make them unsuitable for implementations as sensors. Hooking techniques provide simpler circuits than the analyzers at relatively low frequencies of the resonators; but at high frequencies the circuit complexity increases and the advantages as for simplicity represented with respect to the analyzers or decay techniques are considerably reduced. Consequently, the oscillators become an alternative for monitoring the resonance frequency in high frequency resonators; the low cost, integration capability thereof and the rapid and continuous monitoring of the resonance frequency make them to the selected alternative to implement the QCM sensors at high resonance frequencies. However, in an oscillator, the sensitivity is determined by the stability of frequency and this by the stability of phase, which depends on the phase response of all the components of the oscillator system. In principle, the role of a resonator in an oscillator is to absorb the phase changes occurring in the other components of the oscillating system. The steep slope of the phase-frequency response of the resonator makes these phase changes compensated by with very small variations in the oscillation frequency. However, the variations experienced by the sensor are precisely of interest in the case of a QCM sensor, whereby any variation in the phase response of the other components forming the oscillator circuit will result in frequency instability. Moreover, the quality factor of the resonator as a sensor is greatly reduced in implementations in liquid medium, therefore relatively small changes in the phase response of the other components of the oscillator will result in relatively large variations in the oscillation frequency, which will appear as noise. The frequency and phase noise increase with the frequency of the system, therefore, it is not obvious to say that an increase in the resonance frequency of the sensor will necessarily imply an increase of the sensitivity of the sensor system, as shown in equation (I).
An alternative approach would be to interrogate the sensor with a test signal (so-called test signal) coming from an outside source of great stability in frequency and phase, similarly to what impedance or network analyzers do, but at a test frequency (or frequency test) set within the band of resonance of the sensor. A change in the phase-frequency response of the resonator, for example due to a variation in the weight surface density of the thin layer deposited on the resonator, would be detected from the phase change endured by the test signal. In principle, this phase change should be quantitatively related to the variation in weight on the surface of the sensor. U.S. Pat. No. 5,932,953 granted to Drees et al., claims a method and system based on this idea, which has the following advantages:                The stability of the test signal can be very high so that the precision in the characterization of the response of the sensor is not disturbed by the noise itself of the characterization signal.        The measurement of the lag is performed between the original signal, at the input of the circuit, and the resulting signal affected by the response of the sensor. Therefore, the measurement of lag is differential and any phase instability of the original test signal is simultaneously transferred to the output signal canceling each other out in the differential measurement.        The measurement of the lag can be accomplished with relatively simple circuitry, even at very high frequencies, therefore the system can be implemented by using a simple and easily integrated electronics.        When using a fixed-frequency test signal, the same signal, or a one synthesized therefrom, can be used to simultaneously interrogate other sensors, which greatly facilitates the implementation of systems with multiple resonators.        
However, these apparent advantages that, in effect, could be provided by a measurement method and system based on the original idea of interrogating the sensor device with a fixed-frequency test signal, are never quite achieved by the method and system presented in U.S. Pat. No. 5,932,953 mentioned for the following reasons:
1.—The method claimed in said invention assumes that the measurement of phase provides a quantitative measure of the variation in weight of the sensitive coating deposited on the surface of the resonator; however it provides no mathematical connection between said phase variation and the corresponding weight variation. Therefore, in order to apply said method, a calibration of the sensor device would be necessary; which complicates the application of the claimed method. Moreover, in such patent, it is assumed that the sensitivity given by the connection between the variation of the phase insertion and the weight variation also increases in proportion to the frequency, in the same manner that the connection between the variation in the resonance frequency and the weight variation. This assumption is caused by lack of rigor in the analysis of the problem which intends to be satisfied with the method and system presented in this patent. As discussed in the detailed description of the present invention this is not the case, still more, for resonators in vacuum or gaseous medium, the sensitivity given by the connection between the variation of the insertion phase and the weight variation does not increase in vacuum, and do it slightly in a gaseous medium, by increasing the resonance frequency of the sensor, while in liquid medium it is proportional to the square root of the resonance frequency. This result shown for the first time in the present invention demonstrates that the object thereof is not a simple or trivial modification of the patent before.
2.—The method and system claimed in the U.S. Pat. No. 5,932,953 assume that the frequency of the test signal can be any frequency within the band of resonance of the sensor. As it will be demonstrated in the present invention, this is not true. The test signal which has to be used to establish the phase base or reference line must necessarily be, or be very close to, that so-called “dynamic series resonance frequency” of the sensor (such frequency known as DSRF and defined in the detailed description of the invention); in contrast the measures of the phase variation cannot simply relate to the weight variation, since this connection would depend on the exact frequency of the test signal and the sensor used, which would invalidate any calibration performed at a another frequency and make impractical the implementation of the claimed method. In this sense, the system claims, based on the simultaneous differential measurement of lags produced by two resonators resonance bands of which overlap, one of which is used as reference to cancel the external effects such as temperature, viscosity, etc., and in which the frequency of the test signal is set at the intermediate zone of the overlapping band, does not provide the desired results because the sensors are interrogated in different areas of their phase-frequency response; therefore external effects produce different responses in each resonator, which prevents their cancellation.
3.—Moreover, the selection of the frequency of the test signal, such as revealed in the previous section, has not been scheduled in the claimed method, or the claimed system. Accordingly, the claimed system is not suitable for appropriate measuring the phase variation at the convenient frequency. The system object of the present invention takes into account this aspect, resulting from a rigorous analysis of the problem and, therefore, it is not the result of a simple or trivial modification of the system shown in the patent cited above.
4.—The method and system claimed in the U.S. Pat. No. 5,932,953 only set the measurement of the phase variation. However, the exclusive measurement of the phase variation does not allow ensuring that the phase variations are exclusively related to weight variations in the sensor. Indeed, if the physical properties of the fluid medium on the resonator change, the phase variations can be disrupted by such change by inducing error in the characterization of weight variations. It is thus necessary to include in the system a way that allow establishing the validity of the connection between phase and weight variations.
5.—As mentioned, the phase-frequency sensitivity does not increase with the resonance frequency for the case of vacuum or gaseous medium, even for liquid media it does not increase as much as was expected; therefore it may still be desirable to use the measure of the resonant frequency variation as a characterization parameter. This aspect is not considered by the system claimed in the U.S. Pat. No. 5,932,953 since it has not been revealed until now. The system object of the present invention considers this aspect, after analyzing that the detailed description includes, implementing a feedback system that allows establishing both the appropriate test frequency and the optional measure of the resonance frequency variation.
6.—The U.S. Pat. No. 5,932,953 claims a method and a system wherein the sensor is interrogated with a fixed frequency signal within the band of resonance of the sensor. Once set the test frequency, it remains constant throughout the measurement process. The claimed method and system do not consider the shift suffered by the test frequency, within the resonance area during the measurement process, as a result of the displacement of the phase-frequency curve of the resonator. In addition, it does not provide any procedure for performing the selection of the appropriate test frequency within the resonance area of the sensor. This aspect is very important, as already indicated and as will become apparent in the detailed description of the invention below. the insertion of a controlled feedback that allows fixing the proper frequency of the test signal and, at the same time, determine how the frequency of the test signal is away from its optimum value during the experiment to be monitored, is a nontrivial improvement to the system and method claimed, already presented in the previous section. This aspect is particularly relevant since the modification of the phase-frequency response of the resonator during the experiment may lead to the test signal being eventually interrogating the sensor in a region of its phase-frequency response wherein there is no sensitivity, or this has been greatly reduced, i.e. wherein there is no phase variation exposed to variations in the coating weight; in another way, the response of the sensor is saturated. In particular, in gaseous medium, the saturation of the sensor may rapidly occur, i.e. the excursion of the response between the phase variation and the weight variation can be very short, since the frequency-phase response of the sensor is very abrupt. Therefore, including a method and a system allowing assessing the degree of deviation of the frequency of the test signal, with respect to its optimum value, during the measurement process, and allowing correcting said test frequency in an appropriate and automatic manner when the deviation of the test frequency is above a previously determined value is an important object of improvement.
7.—Finally, the system claimed in the U.S. Pat. No. 5,932,953 merely establishes the measurement of the phase variation of the sensor as a whole. As demonstrated in the detailed description of the present invention, it is necessary to design a system allowing measuring, as accurate as possible, the phase variation due to the change in the response of the mainly associated impedance to the dynamic branch of the sensor. An inadequate design of the system would reduce the sensitivity of the sensor system.
In addition to a suitable electronic characterization method and system, another difficulty to overcome when trying to work with resonators of mainly very high resonance frequency, is their small size and fragility; these features make it extremely difficult to design a measurement cell that meets the following specifications: the electrical contacts of the resonator for connection to the electronic characterization system must be extended and the isolation of one side of the resonator of the fluid medium without excessively disturbing the response of the sensor must be allowed, the conduction of experiments in flow, which a fluid is channeled so as to be in contact with at least one vibrating surface of a piezoelectric resonator has to be facilitated, and a safe handling of the sensor by the experimenters must be allowed. The invention aims to increase the sensitivity of current microbalance systems, whereby it presents both an electronic characterization method and system that must be accompanied by a suitable measurement cell that makes feasible both the application of the method and the electrical characterization of the resonant sensor. Currently there is no measurement cells prepared to work with AT-cut piezoelectric quartz resonators with mainly frequencies above 50 MHz for the reasons mentioned. The present invention provides a support and measurement cell that solves these problems.
The above analysis has served to highlight some key and differential features of the object of the present invention, which are not limited to the mentioned patents but are mostly general to currently existing systems.